Generally, any conventional underwater heater usable for an aquarium uses a main heater body comprising a heat-resistant cylindrical tube internally storing a heating member such as a nichrome wire, a thermal sensor such as a thermistor for detecting water temperature inside of an aquarium, and a parts-mounting substrate for controlling power delivered to the heating member. If water temperature detected by the thermal sensor remains below a predetermined degree, power is fed to the heating member to activate heating operation to raise water temperature therein, whereas if water temperature detected by the thermal sensor exceeds the predetermined degree, power is shut off from the heating member to halt heating operation so that water temperature inside of the aquarium can be lowered.
Nevertheless, if the underwater heater were erroneously operated outside of an aquarium or exposed above water surface caused by decreased water therein during heating operation, the heating member enters into overheating condition to generate abnormally high temperature which can eventually cause fire to break out.
If the aquarium ever overturns as a result of earthquake for example, water flows out of the aquarium causing the heater member to be exposed externally resulting the heating member overheating as discussed above case to which may cause fire to break out.
To deal with the above problem, there is proposed an non-resettable system having a thermal fuse connected to the heating member in series so that the thermal fuse melted whenever the heating member enters into an overheating condition. The thermal fuse cannot return to its original state. Likewise, a resettable system is also known, which is provided with a device of a thermal fuse for controlling operation feeding power to the heating member. If the heating member ever generates abnormally high temperature after entering into an overheated condition, power is shut off via the bimetal device, and after the temperature is lowered, power supply to the heating member is automatically restored.
Nevertheless, according to the former non-resettable system, the heater itself is no longer usable after melting the thermal fuse, and thus, the heater must be scrapped which is wasteful.
According to the latter resettable system, the heater itself is repeatedly usable. But the temperature is raised and lowered repeatedly via the bimetal device, and accordingly it will induce fire to break out as time passes by.